FTTH (Fiber To The Home) using an optical fiber as communication lines that connect computers at home or in an office (home side) to a service provider (central office side) is widely spread. The basic type of the FTTH is P2P (Point To Point), and the central office side and the home side are connected to each other by an optical fiber in a one-to-one relationship. In this case, a line connected to a computer on the home side is a metal line, and a line connected to an upper network on the central office side is also mainly a metal line (note, however, that there may be an optical line case). Therefore, when a metal line and an optical fiber are connected to each other, a media converter serving as a relay apparatus is installed at an end of the optical fiber (see, for example, Patent Literature 1). Each media converter is used in a power-on state all the time, regardless of whether a computer on the home side is performing communication.
Meanwhile, in order to cope with a rapid increase in the number of users using FTTH and to effectively use an optical fiber, a PON (Passive Optical Network) system is used. The PON system is a system in which an optical line terminal serving as a central station is connected to optical network units installed in a plurality of subscribers' houses, by an optical fiber network where a single optical fiber is split into a plurality of optical fibers by an optical coupler (see, for example, Patent Literature 2). When the plurality of optical network units simultaneously perform transmission to the optical line terminal, pieces of transmit data collide with one another. Thus, the optical line terminal provides permission regarding transmit timing and the amount of transmit data, to the optical network units. In response to the permission, the optical network units perform transmission in an upstream direction at the timing and amount permitted by the optical line terminal, to the optical line terminal (see, for example, Patent Literature 3).
Data transmission in a downstream direction from the optical line terminal to the optical network units is multiplexed in a time-division manner. Although transmitted data physically reaches each optical network unit, each optical network unit reads a header portion of a transmitted frame and thereby determines whether the frame is destined therefor (destined for the optical network unit or a user network under the control thereof). If, as a result of the determination, the frame is destined for the optical network unit, then the optical network unit takes in the frame, or otherwise discards the frame. Note that the optical line terminal serving as the central station operates with power-on all the time. Note also that each optical network unit is used in a power-on state all the time, regardless of whether a computer under the control thereof is performing communication.